What Is The Cause And Treatmen For Postnasal Drip?

Hi, I seem to be producing excessive amounts of foamy sticky mucus at the base of my throat, my nose is totally clear, my lungs are fine with good capacity but when ever I sleep, even if I am sitting vertically in a lounge chair, my airways feel like they completely seal over with this mucus, suddenly I wake, often in a panic because I cannot take one breath in or out, I tried using an asthma puffer, kind of helped at first but minimally, doesn't have any effect now, its been two weeks. Sometimes it feels like minutes of battle to swallow this mucus before I finally take a breath, I end up all shaky after the bad attacks, they happen multiple times a night so now I don't sleep hardly at all! Usually I can't talk for up to an hour after a bad attack, I've tried every natural remedy, two courses of antibiotics and now I'm on prednisone, pseudoephedrine and an antihistamine! Any suggestions? P.S I have never smoked in my life!